Change git -P to --no-pager

[jaywonchung]

This pull request changes the -P option given to git to --no-pager in git commit/file/branch previewers because the latter is more backwards compatible.

The -P option was added to git around three years ago as a shorthand for --no-pager (relavant commit: git/git@7213c28). My lab server (remarkably) uses git 2.17.1 and the preview window for git commits displayed nothing. This patch fixed the problem.

Thank you for the great plugin!

[Conni2461]

Thats super good to know thanks :)

I gonna make sure i always do that and update #528 which will introduce new git previewers.

Thanks again :D